Charging device for an energy conversion device

ABSTRACT

A charging device for an energy conversion device, e.g., a fuel cell, of a motor vehicle, has a rotor rotatably mounted on a housing of the charging device, the rotor having a shaft and at least two compressor wheels which are connected in rotationally fixed fashion to the shaft. The compressor wheels have wheel rear parts facing away from respective compressor wheel inlets, by which a medium that is to be supplied to the energy conversion device, e.g., air, is compressible. The wheel rear parts of the compressor wheels are matched to one another such that respective forces which are opposed to one another and which result from respective compressor wheel outlet forces impressed on the wheel rear parts substantially balance one another.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a charging device for an energyconversion device, e.g., a fuel cell.

2. Description of the Related Art

Published Japanese patent application JP 2001/263291 discloses asupporting structure for a high-speed compressor having an electricmotor that includes a rotor shaft mounted by magnetic bearings.

U.S. Pat. No. 6,196,809 B1 discloses a two-stage compressor having twocompressor wheels that are attached to opposite end regions of a shaft.For the axial mounting, magnetic axial bearings are provided that absorbforces in the axial direction of the shaft.

U.S. Pat. No. 6,155,802 discloses a turbo compressor having a first anda second compression chamber, and having a shaft that is connected totwo compressor wheels.

U.S. Pat. No. 6,450,780 B1 discloses a method for producing a gas underpressure using a compressor that is coupled to an electrical machine.The compressor includes a rotor having a shaft that is mounted bymagnetic bearings. In addition, the rotor includes two compressor wheelsthat are connected to the shaft in rotationally fixed fashion.

Published international patent application document WO 03/040567 A1discloses a two-stage compressor having a shaft. The compressor alsoincludes two compressor wheels by which a fluid is to be compressed. Thecompressor wheels are connected to the shaft in rotationally fixedfashion.

The known compressors have further potential for making their operationmore efficient.

Therefore, an object of the present invention is to provide a chargingdevice for an energy conversion device that has particularly efficientoperation.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, a charging device for an energyconversion device, in particular for a fuel cell, of a motor vehicleincludes a rotor that is rotatably mounted on a housing of the chargingdevice, said rotor having a shaft and having at least two compressorwheels that are connected to the shaft in rotationally fixed fashion andthat each have wheel rear parts facing away from respective compressorwheel inlets, by which wheels a medium that is to be supplied to theenergy conversion device, in particular air, can be compressed.

According to the present invention, it is provided that the wheel rearparts of the compressor wheels are matched to one another, so thatrespective oppositely oriented forces resulting from compressor wheeloutlet pressures impressed on the rear parts of the wheels balance oneanother, at least substantially. Due to this balancing of the forcesacting in particular and at least substantially in the axial directionof the shaft, no forces, or only very small forces, have to be absorbedand supported in this direction. In this way, no, or only very small,frictional losses occur that would occur as a result of forces to besupported, so that a very efficient operation of the charging deviceresults. This is also beneficial for a very efficient operation of theenergy conversion device associated with the charging device, so thatsaid energy conversion device can thus be operated in a particularlyenergy-efficient manner, so that the energy converted by the energyconversion device can for example be used in very large part to drivethe motor vehicle, and does not for example remain unused as a result offrictional losses resulting from the described forces.

This is advantageous in particular if the shaft or the rotor is mountedby at least one air bearing, in particular a dynamic air bearing, havingat least one foil for the mounting. As a result of their design, suchair bearings produce higher frictional losses than do other kinds ofbearings such as ball bearings; the predominant part of these frictionallosses, for example two-thirds of the overall frictional losses, occurin particular in the absorption of bearing forces in the axial directionof the shaft.

Through the use of the compressor wheels having the wheel rear partsmatched to one another, it is possible at least largely to cancel theopposed forces, in particular the axial forces, in that the forcescounterbalance one another. A result of this is that a correspondingbearing, in particular an axial bearing, for the absorption of theseforces can be made correspondingly smaller in its dimensions andtherefore less susceptible to losses, or can even be completely omitted.This also results in very low weight as well as, in some cases, a verylow part count of the charging device according to the presentinvention, which on the one hand is beneficial for the efficientoperation of the charging device and on the other hand keeps the costsof the charging device, and therefore of the motor vehicle as a whole,low.

In an advantageous specific embodiment of the present invention, thewheel rear parts are matched to one another with regard to theirrespective diameter. In this way, the balancing of the oppositely actingforces is realized in a particularly simple and economical manner,reducing the complexity of the charging device as well as its partcount, and also keeping low the costs of the charging device. Ifpressures differing from one another act on the wheel rear parts, thenthe respective surfaces of the wheel rear parts on which the pressuresact are correspondingly to be matched to one another with regard totheir surface content, so that from this matching there result opposedforces that balance one another. Here, a design parameter suitable forthe matching of the surface contents is the diameter of the wheel rearparts; for example, a larger diameter results in a larger surface, and acomparatively smaller diameter results in a smaller surface. If, forexample, a first pressure acts on one of the wheel rear parts that isgreater than a second pressure acting on the other wheel rear part, thenthe surface on which the first pressure acts is correspondingly to bemade smaller in its surface content than the surface of the wheel rearpart on which the second pressure acts, so that forces result whosemagnitudes are equal but that are opposed to one another and thereforecounterbalance one another, in particular in the axial direction of theshaft.

During operation of the charging device for compressing the air, inparticular axial forces arise that act in the direction of therespective compressor wheel inlets in the axial direction of the shaft.These axial forces at least substantially counterbalance one another inthe charging device according to the present invention, so that acorresponding axial bearing can be omitted, or at least can be made withsmaller dimensions.

If the charging device according to the present invention is used in amotor vehicle during whose operation there may occur non-steadyoperating states of the energy conversion device and thus of thecharging device, in particular accelerations of the charging device,then a corresponding axial bearing of the rotor may be indispensable inorder to prevent contact between the rotor and, in particular, thecompressor wheels and the housing of the charging device, and thus toensure reliable, long-lived operation of the charging device.

Alternatively or in addition, it can be provided that the rotor ismounted in the radial direction of the shaft by a magnetic bearing,bringing the advantages already described in connection with such amagnetic bearing.

In an advantageous specific embodiment of the present invention, therotor is mounted by at least one air bearing, in particular in the axialdirection. Such an air bearing, which has for example a foil for bearingthe rotor, enables an efficient and reliable bearing of the rotor even,and in particular, at very high rotational speeds of the rotor whichoccur for the efficient compression of the air.

In order to enable the air to be compressed particularly efficiently andso as to meet demand, the charging device has for example a motor, inparticular an electric motor, by which the rotor can be driven. Thisenables an operation of the charging device that is efficient and meetsthe demand for the supply, proportionate to demand, of compressed air tothe energy conversion device, resulting in efficient operation of theenergy conversion device and thus of the motor vehicle as a whole.

In a particularly advantageous specific embodiment of the presentinvention, the compressor wheels for compressing the medium areconnected in series to one another, resulting in a two-stage andparticularly efficient compression of the medium, in particular air.Here, one of the compressor wheels acts as a first compressor stage andthe other compressor wheel acts as the second compressor stage, thediameter of the wheel rear parts, or the diameter of the compressorwheels, being for example correspondingly matched to one another so thatforces that occur during compression, in particular axial forces, canceleach other out at least almost completely, advantageously completely.

It is also possible for the compressor wheels for compressing themedium, in particular air, to be connected parallel to one another. Thisalso enables a particularly advantageous and efficient compression ofthe air; here, compressor wheels can be used that are at leastsubstantially equal with regard to their diameter, or with regard to thediameter of the wheel rear parts, and that differ only in theirdirection of rotation. A stream of the medium, in particular air, thatis to be compressed is then applied simultaneously and in parallel tothese two compressor wheels. In this specific embodiment, a completecompensation is possible of the forces resulting from the compression,in particular the axial forces.

If the rotor has a turbine wheel that is connected in rotationally fixedfashion to the shaft, by which the rotor can be driven, then in this waythe operation of the charging device and in particular of the energyconversion device can be made particularly efficient. Here, for examplethe exhaust gas of the energy conversion device, in particular the fuelcell, can be used to drive the turbine and, via the turbine, thecompressor wheels.

In the charging device according to the present invention, it can beprovided that the compressor wheels are situated in opposite end regionsof the shaft. If the turbine wheel is provided, it is possible for theturbine wheel to be situated between the compressor wheels in the axialdirection of the shaft. It can also be provided that, in the axialdirection of the shaft, first the first compressor wheel is situated onthe shaft and is connected in rotationally fixed fashion thereto, and isfollowed by the second compressor wheel, situated on the shaft andconnected in rotationally fixed fashion thereto, the turbine wheel onlythen being situated on the shaft in the axial direction and connected inrotationally fixed fashion thereto. These specific embodiments areadvantageous and are correspondingly to be selected depending onconditions of space, demands, boundary conditions, and/or the like.

Here it is to be noted that the compressor wheels and the turbine wheelthat may be provided are fashioned as radial compressor wheels or aradial turbine wheel, so that the medium can be compressed particularlyefficiently and the charging device requires only very little space.This solves or avoids packaging problems, in particular inspace-critical areas of the motor vehicle in which the charging deviceis situated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic longitudinal sectional view of a chargingdevice that has a first compressor and a second compressor by which airthat is to be supplied to a fuel cell can be compressed in two stages,the compressor having respective compressor wheels having wheel rearparts that are matched to one another, through which respective, opposedaxial forces resulting from respective compressor outlet pressuresimpressed on the wheel rear parts balance one another, at leastsubstantially.

FIG. 2 shows a schematic longitudinal sectional view of another specificembodiment of a charging device as shown in FIG. 1, whose compressorsare connected to one another in parallel, the air being compressible bythe compressors in one stage.

FIG. 3 shows a schematic longitudinal sectional view of a furtherspecific embodiment of the charging device in FIG. 2, the chargingdevice having a turbine capable of driving the compressor forcompressing the air.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a charging device 10 that is allocated to a fuel cell andby which air that is to be supplied to the fuel cell can be compressed.The fuel cell uses this compressed air supplied to it to convert theoxygen in the air, and the hydrogen supplied to it, into electricalenergy.

Charging device 10 has a first compressor 12, fashioned as a radialcompressor, having a housing 14 in which a first compressor wheel 18,having a first wheel rear part 16, is accommodated. In addition,charging device 10 has a second compressor 20 fashioned as a radialcompressor, having a housing 22 in which a second compressor wheel 26having a second wheel rear part 24 is accommodated.

Compressor wheels 18 and 26 are situated in respective end regions 28and 30 of a shaft 32, on said shaft, and are connected in rotationallyfixed fashion thereto, charging device 10 having a rotor 34 to whichcompressor wheels 16 and 26 and shaft 32 are allocated.

In addition, charging device 10 has an electric motor 36 by whichcompressor wheels 18 and 26 for compressing the air can be driven viashaft 32. For this purpose, shaft 32, as well as compressor wheels 18and 26, rotate about an axis of rotation 51 at very high rotationalspeeds.

The air that is to be compressed is supplied to compressor 12, acting asthe first compressor stage, in the direction shown by arrow 40, the airflowing to the corresponding compressor wheel 18 via a compressor wheelinlet 42, being compressed by compressor wheel 18, and flowing out ofcompressor wheel 18 via a compressor wheel outlet 44, into a channel 46.Via channel 46, the air pre-compressed by compressor wheel 18 is guided,in the direction shown by arrow 48, to compressor 20, acting as thesecond compressor stage. The pre-compressed air flows to compressorwheel 26 via a corresponding compressor wheel inlet 50, is compressed bycompressor wheel 26, and flows out of compressor wheel 26 via acorresponding compressor wheel outlet 52 and into a correspondingchannel 54, via which the further compressed air is finally supplied tothe fuel cell in the direction shown by arrow 56.

In order to keep frictional losses of charging device 10 low, and thusto realize a particularly efficient operation thereof, wheel rear parts16 and 24, and thus compression wheels 18 and 26, are matched to oneanother with regard to their diameter, whereby opposed forces resultingfrom compressor wheel outlet pressures impressed on each of wheel rearparts 16 and 26 balance one another at least substantially. These forcesare axial forces and act in the axial direction of rotor 34 or of shaft32, in the direction shown by arrow 58, and are indicated in FIG. 1 byarrows 60 and 62. Due to their direction of action in the axialdirection, these forces are indicated as axial forces in the directionof arrow 58.

As can be seen in FIG. 1, the axial forces indicated by arrows 60 and 62are oriented in opposite directions and act in the direction of therespective compressor wheel inlets 42 and 50 via which air flows tocompressor wheels 18 and 26.

On the basis of the at least substantial balancing of the axial forces,a bearing of rotor 34 for the absorption of these axial forces can beomitted, or can be made particularly small in its dimensions, so thatno, or only very small, frictional losses occur as a result of anabsorption of the axial forces.

Due to the two-stage compression of charging device 10 shown in FIG. 1,the compressor wheel outlet pressures prevailing at correspondingcompressor wheel outlets 44 and 52 and impressed on wheel rear parts 16and 24 differ from one another, so that surfaces differing from oneanother on which the compressor wheel outlet pressures act are fashioneddifferently from one another as a result of a different realization ofthe corresponding diameters.

In contrast to charging device 10 shown in FIG. 1, the charging deviceshown in FIG. 2 realizes a two-stage compression of the air that is tobe supplied to the fuel cell. Compressors 12 and 20, or compressorwheels 18 and 26, are here not connected in series to one another as inFIG. 1, but rather are connected parallel to one another. This meansthat air that is to be compressed is supplied in the direction of anarrow 64 to compressors 12 and 20, or compressor wheels 18 and 26, inparallel fashion via the respective compressor wheel inlets 42 and 50.

Compressors 12 and 20 thus compress the supplied air in parallelfashion. Correspondingly, the air compressed in parallel is also led outvia channels 46 and 54 in the direction of an arrow 66 and is suppliedto the fuel cell. In the one-stage and parallel compression of the airusing charging device 10 shown in. FIG. 2, as a result of the compressorwheel outlet pressures impressed on wheel rear parts 16 and 24, axialforces (arrows 60 and 62) result that balance one another due to thematching of wheel rear parts 16 and 24 to one another. In the one-stagecompression shown in FIG. 2, it is possible to use compressor wheels 18and 26 that are at least substantially identical and that differ fromone another only with regard to their direction of rotation forcompressing the air. The two compressor wheels 18 and 26 of chargingdevice 10 shown in FIG. 2 thus share an air mass flow that is to becompressed and that is to be supplied in the direction of arrow 64,enabling an at least nearly complete compensation of the axial forces.

Compressors 12 and 20 compress the air to an at least almost identicalpressure level, so that the compressor wheel outlet pressures acting onwheel rear parts 16 and 24 are at least substantially equal.Correspondingly, identical surface contents of wheel rear parts 16 and24 on which the compressor outlet pressures act are sufficient for theat least substantial balancing of the axial forces.

If charging devices 10 shown in FIGS. 1 and 2 are used for example in amotor vehicle, in particular a passenger vehicle, then during operationof the motor vehicle non-steady operation of charging devices 10 mayoccur, in which rotor 34 has to be alternately accelerated, braked,accelerated again, etc. As a result of this non-steady operation,despite the corresponding matching of wheel rear parts 16 and 24 to oneanother (for the balancing of the axial forces in at least approximatelysteady operation), in some circumstances axial forces may occur that donot balance one another. In some circumstances, this then requires anaxial bearing of rotor 34 for, if warranted, a very short-durationabsorption and supporting of forces acting in the axial direction alongarrow 58. Because, however, these forces may occur only for a very shorttime due to the matching of wheel rear parts 16 and 24, and theirmagnitude is small, such an axial bearing can be made small with regardto its dimension and its weight, so that only small frictional lossesresult from the absorption of these forces, and in addition aparticularly efficient operation of charging device 10 is ensured.

FIG. 3 shows another specific embodiment of charging device 10 shown inFIG. 2, rotor 34 of charging device 10 having a turbine wheel 68 that isconnected in rotationally fixed fashion to shaft 32 and is accommodatedin a housing 70 of a turbine 72 of charging device 10.

Turbine 72 with turbine wheel 68 is used to supply exhaust gas from fuelcell 10, via a channel 74 provided through housing 70, to compressorwheel 68, and to drive compressors 12 and 20, or correspondingcompressor wheels 18 and 26, in order to compress the air.

According to charging device 10 shown in FIG. 2, compressors 12 and 20compress the air in parallel and in one stage, the air being supplied tocompressors 12 and 20 in the direction of arrows 76 via a correspondingchannel 78. Via channel 78, the air to be compressed flows to compressorwheels 12 and 20 via corresponding compressor wheel inlets 42 and 50.

When the air is compressed, the air in compressor wheels 12 and 20 flowsout as shown via compressor wheel outlets 44 and 52 and is conducted tothe fuel cell via channels 46 and 54, in the direction of an arrow 80.

As can be seen in FIG. 3, compressor wheel 18 is also situated in endregion 28 of shaft 32. In end region 30 of shaft 32, however, there issituated not compressor wheel 26 but rather turbine wheel 68, which isconnected in rotationally fixed fashion to shaft 32. Compressor wheel 26is situated, in the axial direction of rotor 34 or of shaft 32, betweencompressor wheel 18 and turbine wheel 68, in an intermediate region 83of shaft 32, and is connected in rotationally fixed fashion thereto.

In order to absorb the axial forces that are shown and that occur inparticular during non-steady operation, and that do not balance andcompensate one another, in FIG. 3 an axial bearing 82 is shown that iscapable of absorbing and supporting the axial forces, thus preventingundesired contact of rotor 34, and in particular compressor wheels 18and 26, as well as turbine wheel 68, with housings 14, 22, and 70.

Axial bearing 82 shown in FIG. 3, which can absorb both axial forces inthe direction of arrow 60 and those in the direction of arrow 62, isfashioned for example as an air bearing.

What is claimed is:
 1. A charging device for a fuel cell of a motorvehicle, comprising: a housing; and a rotor rotatably mounted on thehousing, wherein the rotor includes a shaft and at least two compressorwheels connected to the shaft in rotationally fixed fashion, whereineach compressor wheel has a wheel rear part facing away from arespective compressor wheel inlet, wherein the at least two compressorwheels compress air which is to be supplied to the fuel cell, andwherein the wheel rear parts of the two compressor wheels are matched toone another such that opposing forces resulting from respectivecompressor wheel outlet pressures impressed on the wheel rear partssubstantially balance one another.
 2. The charging device as recited inclaim 1, wherein the wheel rear parts are matched to one another withregard to their diameter.
 3. The charging device as recited in claim 2,wherein the rotor is mounted by at least one air bearing in the axialdirection.
 4. The charging device as recited in claim 2, wherein the atleast two compressor wheels for compressing air are connected in series.5. The charging device as recited in claim 2, wherein the at least twocompressor wheels for compressing air are connected in parallel.
 6. Thecharging device as recited in claim 2, further comprising: a drive motorconfigured to drive the rotor.
 7. The charging device as recited inclaim 2, wherein the rotor has a turbine wheel connected in rotationallyfixed fashion to the shaft, and wherein the rotor is driven by theturbine wheel.
 8. The charging device as recited in claim 7, wherein theturbine wheel is driven by an exhaust gas of the fuel cell.